The present invention relates to an inspection method and apparatus for a mask pattern on a photomask or a wafer used in the fabrication of a semiconductor device such as an integrated circuit (IC) or a large scale integrated circuit (LSI).
For the sake of convenience, the short term "IC" will be used as a "semiconductor integrated circuit device" in this disclosure hereinafter.
In an IC patterning process, there are two ways to print a mother pattern called a "reticle pattern" on a wafer. One is a technique of applying a photomask, and the other is to print the reticle pattern directly on the wafer. FIGS. 1 and 2 show the patterning processes in prior art IC fabrication with FIG. 1 for the former and FIG. 2 for the later. In the technique of FIG. 1, the IC pattern of the photomask is first made from the reticle pattern by a step and repeat printing method, and then the IC pattern of the photomask is directly printed on the wafer. In the technique of FIG. 2, the reticle pattern is directly printed on the wafer by a step and repeat method without making the photomask.
In FIGS. 1 and 2, the reticle 100 is made of a piece of silicate glass for example, on which a mother pattern is printed photographically from an original pattern. The mother pattern 101 is called a "reticle pattern" herein. As the reticle pattern 101 is the mother pattern, the pattern must be made with high accuracy, and therefore, the size of the pattern is as large as 5 to 10 times the actual IC size. The reticle pattern is printed on the photomask or the wafer by an optical system having a reduction factor of the value between 1/10 and 1/5.
In FIG. 1, the reticle pattern 101 of the reticle 100 is exposed on a photographic plate fabricated on the surface of the photomask 200 by an optical system 105 having a reduction factor of the same value between 1/10 and 1/5. The exposure is made by a step and repeat procedure moving the photomask 200 in X and Y directions. An individual IC pattern 201 is on the photomask 200, and its size is equal to that of an IC pattern on the wafer. Therefore, after fabricating the photomask, the photomask pattern having a plurality of the IC patterns is exposed on the surface of the wafer 300 in equal size; that is, the size of an IC pattern 301 on the wafer 300 is equal to that of the IC pattern 201 on the photomask 200. An arrow 205 shows this direct printing without reduction or magnification.
In FIG. 2, the reticle pattern of the reticle 100 is exposed on the surface of the wafer 300 by an optical system 305 having a reduction factor between 1/10 and 1/5 for the same reason mentioned above in FIG. 1. The exposure for printing is also made by the step and repeat procedure similar to the technique of FIG. 1, by moving the wafer 300 in the X and Y directions. An individual IC pattern 301 is on the wafer 300.
The term "printed pattern" will be used hereinafter for the printed IC patterns on the photomask 200 or the wafer 300 in FIG. 1, and on the wafer 300 in FIG. 2. Whereas, the term "mask pattern" will be used for the IC patterns on the photomask 200 in FIG. 1, and the reticle pattern 101 on the reticle 100 in FIG. 2. Similarly, as the reticle pattern itself is made by printing an original pattern as mentioned before, the original pattern becomes a mask pattern and the reticle pattern 101 on the reticle 100 becomes a printed pattern in this case.
The exposing process for the printing is very important in the IC patterning process, and a defect can not be allowed to exist. However, it has become necessary to pay great attention to the fact that recently a high probability of incorrect printing has tended to occur because the IC pattern has become very small and complicated to increase the IC packing density.
FIG. 3 shows examples of defects in the printed patterns. FIG. 3(a) shows an original pattern of the reticle pattern, FIG. 3(b) shows an example of a defect in the printed pattern (reticle pattern) on the reticle, and FIG. 3(c) shows another example of a defect in the printed reticle pattern. In FIG. 3(b), the printed pattern 1001 has an incorrect part 1011 which is produced primarily by an incorrect developing process. In FIG. 3(c), the printed pattern 1002 has a defective part 1012 which is produced primarily by an incorrect exposing process.
Defects of these types will also occur in the patterning process of the wafer, and in each case, the defect occurs in the exposing or printing process presupposing that the mask pattern is correct. Therefore, it can be said that the defect can be avoided by paying attention to the semiconductor patterning process. However, if the mask pattern itself has a defect or dust exists near by the mask pattern, an abnormality on the printed pattern cannot be avoided.
FIG. 4 illustrates an example showing the same patterning process as FIG. 2; that is, the wafer is printed directly from the reticle pattern of the reticle. In FIG. 4, a reticle 12, a wafer 16, and an optical system 20 having a reduction factor of the value between 1/10 and 1/5 are shown. A reticle pattern 60 and a piece of dust 14 are shown on the reticle 12. If the dust 14 exists on the reticle 12, a dust pattern 141 is printed on the wafer 16 beside a correct printed pattern 18 of the mask pattern 60. FIG. 4 (b) shows an expanded perspective illustration of the printed pattern on the wafer 16. Though it is not shown in the drawings, the dust pattern 141 is printed on all the patterns on the wafer 16.
If dust is on the mask pattern or if the mask pattern itself has a defect, the whole patterning process ends in a failure. If such problems occur, the IC product suffers damage even though every later process is correct. This has a great influence on the IC cost, because the IC patterning process becomes much too complicated as the packing density increases. Thus, the cause of the defect must be found and removed as quickly as possible in the early stage of the patterning process.
Usually, the reticle pattern itself can be carefully inspected by various methods. Therefore, a problem caused by defects of the reticle pattern itself can be avoided. However, when a reticle pattern, which has been inspected and judged to have no defects, is printed on the wafer, still the following problems may occur.
First, when the reticle is mounted on a projecting system as shown by the reticle 12 in FIG. 4(a) and dust happens to be stuck on the reticle, the image of the dust is printed. Second, when the optical system has some defect, the reticle pattern can not be correctly printed on the wafer.
Thus, though the reticle pattern itself is perfect, the above problems are produced after reticle inspection, and cause a defect to the actual optical image of the reticle pattern projected on the wafer. The optical image can not be detected by the prior art reticle inspection.